For generations, people of every age have dreamed about planet-hopping in starships equipped with faster-than-light drives. The stories that entertain us have evolved from armor-clad knights on horseback to force field-shrouded knights who battle aliens from other dimensions. We don’t want to fly only to another country; these days, people want to step into a parallel universe to defeat their evil twin and rescue the princess or prince. Or how about a world where princesses and princes can’t be differentiated by biology because gender is flexible?

The good news is that although some of these dreams are unlikely, all are scientifically possible by extrapolating from today’s technology.

Remember, science tells us what is, not what we want. Science fiction has no such restriction. Here’s a bit of science fun from my book Blockbuster Science: The Real Science in Science Fiction. I hope it gives you some fun fiction ideas.

Science fiction books, movies, and television shows get a lot of mileage out of driving their characters through space and time. I’m sure you know that space travel is a fact, but did you also know time travel is possible? It is, but before we drop down that rabbit hole, I have a simple question for you. What is time?

Not so easy to answer, is it? I think most people would agree with St. Augustine’s answer: if no one asks me, I know what it is.

I submit to you that time may be an illusion and not fundamental to the universe. Granted it is very real in our limited day-to-day domain. However, there is no universal now. Now is like up and down—a local thing. On the other side of the globe, a person’s down is your up. There is no universal up nor is there a universal now.

In our universe, as one hour passes for you it is possible that 10 has passed on someone else’s clock. Even though nows can differ they are linked to a constant—the speed of light. Albert Einstein summarized this idea in his theories of relativity where he, spectacularly, revealed time to be woven into the fabric of space. Yes, space and time are so intertwined in the physics of the universe that they must always be considered together. Physicists came up with a creative name for this unification: spacetime. Punchy, right? The unification is also sometimes referred to as the space-time continuum.

Here is a weird truth, the reason we can’t move faster than the speed of light is that we are always moving through spacetime at the speed of light. Yes, our velocity through time added to our velocity through space always equals the speed of light. Some cool math hangs around behind this, but rather than blinding you with flashy equations, I’ll tell you their conclusion: the faster we travel through space, the slower our journey becomes through time. In other words, as our speed increases, time bends to conserve the speed of light. This time-bending is called time dilation. Don’t worry, at our earthly speeds the time effects are minimal. Only after we have accelerated our starships to high speeds do the relativistic effects on time become more severe.

Time dilation is a great scientific tool for the science fiction toolbox. When the science in fiction is accurate, characters traveling at high velocities through space should be experiencing (suffering from, really) time dilation. Two examples of relativistic time travel in science fiction stand out. The first is Forever War by Joe Haldeman; the second is Orson Scott Card’s Ender’s Game.

In Forever War, time dilation pops in when troops travel in speedy ships to military encounters on different planets. Each battle takes our hero William Manella centuries farther from the earth he knows. When eventually he returns home, he is so socially displaced that his language is archaic and his heterosexuality is repulsive. Try to imagine two races fighting each other hindered by time dilation, unaware of the enemy’s stage of development when they next engage. It really is all relative!

Ender’s Game also revolves around a relativistic war. The humans have charmingly named their alien enemies buggers. A secondary character, a hero from an earlier confrontation with the buggers, has been stashed away in the spaceship (eighty years before the book’s setting) and sent on a journey at near light speed. He will return, nearly unaged, to earth when the humans have their fleet ready for the final battle against the buggers.

Gravity also affects time. Massive objects such as planets not only stretch space but also stretch time. The steeper the curve in spacetime, the slower time flows. Your position in space relative to massive objects changes the rate you experience time passing. The closer to the Earth’s core, the slower clocks tick. In fact, the Earth’s core is 2 ½ years younger than its surface.

A fictional example of gravitational time dilation can be found in the movie Interstellar. Joseph Cooper, the main character, spends a short time in the gravity well of a black hole. Eighty years have passed on Earth by the time he returns. Oops.

So far, I’ve only told you about jumping into the future but what about travel into the past? Science does allow for it. So, why haven’t we met any time travelers from the future?

Think of time travel as a journey along a two-lane highway that contains many on- and off-ramps. As a time traveler, you can’t visit an earlier era unless an exit ramp to that era exists. This gives us the following scientific rule of time travel: it is impossible to travel back to a time before time machines exist. This rule comes from Einstein’s equations for general relativity where solutions for the continuum must exist at both ends, no matter whether we are talking about location or time.

But, for the sake of argument, if an alien race invented a time machine, say, a thousand years ago, and we got our greedy hands on the machine, it might be possible to travel a thousand years into our past.

Goodbye for now. I hope to meet you somewhere in time and space.